MC still plays MKV files just fine. Has for a very long time, and the very latest version still does. Perhaps a reboot of your system, and if necessary a re-install of MC would help? Otherwise, more information required. For example, do none of your MKV files play, or do just some have problems? Do all other formats play okay? Have you changed or installed anything new recently? etc.
The forum software was changed a few months back, but I haven't heard of anyone losing the login ID. Did you try to do a password recovery on the old account? Maybe one of the JRiver staff will answer this question with more information.
